In the fields of iron and steel, nonferrous metals, cement, incinerators, ash melting furnaces, etc., refractory products are widely employed, and a zirconia-mullite refractory raw material is commonly used as a raw material for the refractory products. Particularly, it is widely used as a refractory raw material for a plate brick or a nozzle to be used in a sliding nozzle device for controlling a flow volume of molten steel during continuous casting of steel.
Generally, a zirconia-mullite refractory raw material is industrially produced through a fusion process designed to melt a mixture of zircon and alumina or a mixture of zirconia, silica and alumina, using an electric arc furnace or the like. The zirconia-mullite refractory raw material comprises a mineral phase consists mainly of crystalline zirconia and mullite, wherein crystal grains of the crystalline zirconia are dispersed in a microstructure of a refractory product to prevent the development of a crack. In addition, it is considered to be excellent in thermal shock resistance because of its low thermal expansion rate as compared with other refractory raw materials such as alumina.
In typical zirconia-mullite, crystal grains of crystalline zirconia include a relatively large crystal (primary zirconia crystal) having a grain size of about 100 μm, which is precipitated as a primary phase during cooling after melting, and a relatively small crystal (eutectic zirconia crystal) having a grain size of about 10 μm or less, which is precipitated at a eutectic point in a last stage of the cooling. As for the primary zirconia crystal to be precipitated as a primary phase, after a seed crystal is precipitated in an initial stage of the cooling, the crystal will grow along with the cooling, so that it is formed as a relatively large crystal having a grain size of about 100 μm. As for the eutectic zirconia crystal to be precipitated at a eutectic point, a liquid phase is crystallized in the last stage of the cooling at a time, and thereby crystal growth is not promoted, so that it is formed as a fine crystal having a grain size of about 10 μm or less.
As for the mullite, precipitation is initiated after the primary zirconia crystal is precipitated, and the precipitated crystal grows up to about 100 μm along with the cooling. A matrix glass exists to fill each gap between the crystals.
In such a microstructure, the primary zirconia crystals and the mullite exist as relatively large crystals, so that the matrix glass as a matrix part thereof consisting primarily of SiO2, and the gap, become larger.
For example, as a refractory product using such a zirconia-mullite refractory raw material, a refractory product for continuous casting is described in the following Patent Document 1, which uses a zirconia-mullite refractory raw material having a mineral phase consisting primarily of mullite and baddeleyite (crystalline zirconia), and comprising, as chemical components, 30 to 80 mass % of Al2O3, 10 to 65 mass % of ZrO2, and 5 to 25 mass % of SiO2. The refractory product using this zirconia-mullite refractory raw material is described as having a low thermal expansion rate and excellent corrosion resistance.
Further, a refractory product for continuous casting is described in the following Patent Document 2, which contains electro-fused alumina-zirconia, wherein an average diameter of primary alumina crystals is in the range of 5 to 70 μm, and zirconia is contained in an amount of 5 to 43 mass %. It is described therein that, if the average diameter of alumina crystals to be precipitated as primary phases is set in the range of 5 to 70 μm, the primary alumina crystals and alumina-zirconia eutectic crystals will be mixed finely and complexly, so that energy required for cracking an electro-fused alumina-zirconia grain becomes larger, and thereby thermal shock resistance is improved as compared with a conventional alumina-zirconia raw material.
A fused alumina-zirconia-silica based refractory material is described in the following Patent Document 3, which has a basic microstructure consisting of corundum crystals, baddeleyite crystals (crystalline zirconia) and a matrix glass, wherein the refractory material contains: as chemical components and in mass % on the basis of oxide, ZrO2 in an amount of 25 to 32%; Al2O3 in an amount of 55 to 67%; SiO2 in an amount of 5 to 12%; P2O5 in an amount of 0.05 to 0.5%; B2O3 in an amount of 0.05 to 0.5%; and Na2O and K2O in an individual amount of 0.1 to 0.5% and in a total amount of 0.6% or less. It is described therein that Na2O and K2O as chemical components are contained in an individual amount of 0.1 to 0.5% and in a total amount of 0.6% or less to achieve an effect of suppressing leaching of a matrix glass consisting primarily of SiO2 during use under a high temperature of 1400° C. or more.    [Patent Document 1] JP 56-96775A    [Patent Document 2] JP 2000-44327A    [Patent Document 3] JP 10-101439A